Formerly included in the genus Hexaprotodonbut a review of the taxonomy and phylogeny of the hippopotamids has restricted the definition of Hexaprotodon to extinct Indian and Southeast Asian Hippos and revalidated Choeropsis for the extant Pygmy Hippo (Boisserie 2005).

Two subspecies are recognized. The nominate subspecies C. l. liberiensis occurs in Côte d’Ivoire, Guinea, Liberia and Sierra Leone. The subspecies C. l. heslopi (Heslop’s Pygmy Hippopotamus)is known only from the Niger Delta,Nigeria. It was considered a distinct subspecies by Corbet (1969) and Coryndon (1977), based on variations in cranial anatomy.

Published figures on population size are contradictory, with some reports from Côte d'Ivoire indicating that numbers are probably higher than pre-existing estimates (Robinson 2013). However, evidence from camera trapping and sign surveys indicates that densities are low, particularly in key sites, such as Sapo National Park, Liberia. Large areas of the original forest habitat, especially in Côte d'Ivoire, have been destroyed or degraded by commercial plantations of oil palm and other products, shifting cultivation, mining and logging, and hunting for bushmeat is increasing throughout the range (Mallon et al. 2011, FFI and FDA 2013). Even if the estimate of 2,000-3,000 used previously was doubled to 4,000-6,000, using the lower end of the range (4,000), on a precautionary basis, suggests that the number of mature individuals is still <2,500. A continuing decline of around 20% over two generations (26 years; Pacifici et al. 2013) is estimated based on the extent of forest loss across the range in combination with an increase in hunting intensity, hence the species qualifies for listing as Endangered under criterion C1.

The nominate subspecies is endemic to the Upper Guinea Forest of West Africa, occurring in four countries, Côte d'Ivoire, Guinea, Liberia and Sierra Leone. The second subspecies, C. l. heslopi, formerly occurred some 1,800 km to the east, on the other side of the Dahomey Gap, in Nigeria from the Niger Delta east to the Cross River in Nigeria (Heslop 1945; and see Corbet 1969, Ansell 1972). There have been no reliable reports of this subspecies since 1945, and its continued presence seems unlikely (Robinson 2013).

The historic distribution of the Pygmy Hippo was far more extensive than the distribution today. Populations have disappeared from many sites and become fragmented across the landscape. There are confirmed recent records from localities in each of the four range countries and additional sites that have not been surveyed in recent years may still harbour Pygmy Hippo populations (Mallon et al. 2011). Full details of the current distribution are therefore unknown but a best assessment is provided by Mallon et al. (2011).

Liberia is at the centre of the species’ range and has the most extensive tracts of intact lowland forest in the region (Christie et al. 2007), with occurrence in the other three range countries primarily close to their borders with Liberia: eastern Sierra Leone, south-east Guinea, and south-west Côte d’Ivoire (Eltringham 1993, 1999; Grubb et al. 1998; Roth et al. 2004; Mallon et al. 2011).

In Sierra Leone, Pygmy Hippo populations are found in the Gola Forest region bordering Liberia, around the Loma Mountains in the north of the country and along the Moa River, including Tiwai Island (Mallon et al. 2011). There are still rumours of Pygmy Hippos in the Outamba-Kilimi National Park in north-west Sierra Leone where it is possible they may survive in sympatry with the Common Hippo but there is no reliable evidence to confirm this.

The Republic of Guinea has a fragmented Pygmy Hippo population occurring in the forest zone of the south-east. There are records since 2000 from the Ziama Biosphere Reserve, Diécké Forest Reserve, Mont Béro Reserve, and also in Tinzou Community Reserve. Pygmy Hippos formerly occurred in Déré Forest in the extreme south on the border with Liberia but a survey carried out by the NGO Sylvatrop in 2009 found no evidence of Pygmy Hippo presence (Mallon et al. 2011). No evidence of Pygmy Hippos has been found in the Guinea part of the Cavally River (Robinson 2013).

In Côte d’Ivoire the most important site for Pygmy Hippos is Taï National Park and its adjacent zone of protection including N’Zo Faunal Reserve (Roth et al. 2004). They are also reported to be present in the Goin Débé Classified Forest, Cavally Classified Forest on the border with Liberia and Azagny National Park in the south-centre of the country (Mallon et al. 2011).

In Liberia the Pygmy Hippo population is divided between the two large remaining blocks of forest in the southeast and northwest of the country. The population in the southeast is centred on Sapo National Park (Collen et al. 2011) with recent records along the Duobe River to the north of Sapo across to the Grebo National Forest on the border with Côte d'Ivoire, as well as along Kia Creek in Maryland/River Gee counties and within the proposed Grand Kru-River Gee Protected Area (Mallon et al. 2011). Pygmy Hippos are still likely to occur in other forests between the Cestos and Senkwehn rivers, where abundant signs of their presence were found in 1998 (Robinson and Suter 1999). In the northwest, there are recent records along the border with Sierra Leone in the Gola National Forest and in the Wonegizi National Forest on the border with Guinea (Mallon et al. 2011).

Records of the species from Gambia and Ghana were rejected by Grubb et al. (1998), while another isolated record from Guinea-Bissau (Cristino and Melo 1958) almost certainly refers to the Common Hippo (Robinson 2013).

The total size of the wild population is unknown. The 1993 IUCN Status Survey and Action Plan estimated the population at "a few thousand at most" (Eltringham 1993). This estimate was cited as "2,000-3,000" in the 2008 IUCN Red List assessment (Lewison and Oliver 2008) who suggested that even that figure may be too high.

However, some much higher population estimates, originally published in GTZ reports (Hoppe-Dominik 1999) and in university theses (Bülow 1988, Hentschel 1990), have been made for Côte d’Ivoire. These have been assessed and summarised by Roth et al. (2004).

Density estimates were calculated from track and dung counts made along fixed transects during a long-term monitoring programme in Taï National Park. Hentschel (1990) estimated mean densities of 3.6 individuals/km2 in primary forest, with a maximum of 7.6/km2 in one locality, and 2.9/km2 in secondary forest. On the basis of these figures, Roth et al. (2004) concluded that there may have been 10,000 Pygmy Hippos in Taï National Park and 19,000 in Côte d’Ivoire during 1982-1986, but that numbers in Taï had fallen to 5,000 by 1997. Densities in the east of Taï fell to 0.3/km2 in 1998 and 0.2/km2 in 2001 and in the west to 0.8/km2 in 1995 and 1.4/km2 in 1998; while the latest figures showed densities in the best areas of 1.4-2.5/km2 (Hoppe-Dominik 1999, Roth et al. 2004).

The present size of the global population is unknown, but it is widely considered to be declining based on the evidence of rate of conversion of forest habitat in the range, reports of poaching and the reduced densities in Taï National Park reported by Roth et al. (2004).

The Pygmy Hippo is rarely seen because of its secretive, nocturnal habits and consequently not much is known of its ecology. The most detailed field studies are those by Robinson (1970), Bülow (1988) and Hentschel (1990). A general account of its biology is given by Eltringham (1999) and Robinson (1981b) compiled a detailed bibliography of the species.

The Pygmy Hippo is solitary except when a female is accompanied by young or during the brief association of a breeding pair (Robinson 1970, 1996; Robinson and Suter 1999). They are primarily, but not exclusively, nocturnal. Radio-collaring evidence showed that in Azagny National Park they were active mainly from late afternoon until midnight with peak activity between 16:00h and 23:00h (Bülow 1988). Recent camera trapping in Liberia and Sierra Leone has revealed that they may also be active throughout the night and also during the day. They spend the day hidden in swamps, wallows or hollows under the banks of streams (Robinson 1981a, Roth et al. 2004) or on drier ground within swamps (Bülow 1988). Pygmy Hippos follow well-defined trails or tunnel-like paths through the forest and swamp vegetation, which they mark by spreading dung by vigorously wagging their tail while defecating (Johnston 1906, Robinson 1970, White 1986).

It mainly inhabits lowland primary and secondary forests, close to rivers, streams and Raphia palm tree swamps (Robinson 1970, Bülow 1988, Eltringham 1999), sometimes being found along gallery forests extending into Transitional Woodland and the southern Guinea savanna. The habitat characteristics which appear to be most important are the presence of small streams with submerged trees, root hollows, swampy depressions, and the size and density of ground vegetation (Roth et al. 2004).

Pygmy Hippos feed on terrestrial and semi-aquatic plants. In the wild, they are known to eat ferns, tender roots, grasses, herbs, stems and leaves of young trees, vegetables and fallen fruit; they have also been observed to eat sweet potato leaves, okra, pepper plants, cassava and the tender shoots of young rice plants on plantations and farms at the forest edge (Robinson 1970, 1996; Bülow 1988; Hentschel 1990), but are not regarded as crop pests. Robinson (2013) consistently found that a small, recumbent, vine-like forb, widely known as ‘Deewinkon’ (Geophila sp.) is a preferred food throughout Liberia.

Pygmy Hippos have been reported to feed for 5.8-6 hours per day, between mid-afternoon and midnight (Robinson 1981a, Eltringham 1999) but camera trapping has shown they may feed throughout the night. They may defoliate young trees by taking the base of the twig in its mouth, then shaking its head while pulling the twig through the mouth and they have also been observed standing on their hind legs with their front legs against the stem to reach ferns growing within Raphia palms (Bülow 1988). White (1986) recorded Pygmy Hippos taking fish from traps in Sierra Leone.

The stomach has four chambers (Langer 1988). The first three are covered with tough keratinized epithelium, only the last containing glandular epithelial tissue. There is evidence that microbial breakdown of plant material takes place in the first three stomach chambers, no caecum being present in this species. This mode of digestion is usually considered an adaptation to a highly fibrous, generally "low-quality" vegetable diet. The droppings are poorly formed and similar to those of the Common Hippo. Heaped dung and tail-splattered excrement are commonly found along trail-side vegetation.

Little is known about Pygmy Hippo ranging patterns, home range size or territoriality. Bülow (1988) radio collared five animals (four females, one male) in Azagny National Park, Côte d’Ivoire, and tracked them for 3-6 months (Bülow 1988). This study found that female home ranges overlapped and estimated that they covered 40-60 ha, while the home range of the male covered 150 ha. Bülow (1988) also found that the male Pygmy Hippo covered a distance of two km per day, whereas the longest recorded distance moved by a female was 900 m. Home ranges seem to depend on the presence of small streams with submerged trees, root hollows, swampy depressions, and size and density of ground vegetation, rather than nutritional factors or proximity of rivers (Roth et al. 2004). During the rainy season (May–September), animals are reported by hunters to disperse over wide areas in the forest zone.

No accurate data on reproduction, including breeding season, have been published for the wild populations. Sexual maturity occurs at about four to five years of age. From studies of captive animals (Lang 1975, Tobler 1991), the oestrous cycle has been shown to average 35.5 days with oestrus itself being 24–48 hours long. The average gestation length is 188 days. A single young is born with a birth-weight of 4.5–6.2 kg. Twins are born very rarely the incidence being approximately one in every 200 births (Hlavacek et al. 2005) The young are born on land and there is no evidence from captive births that a nest is constructed. A survey of over 800 births indicates that these occur throughout the year (Tobler 1991) although Robinson (1970) reports hunters observing newborns more commonly in the early dry season between November and January. Weaning occurs at 6–8 months. Reproductive maturity is reported to be 3–5 years (Lang 1975). Captive adult body size is achieved by three years. Maximum age in captivity is approximately 35–40 years but no while no data exist for wild animals.

In captivity, copulation has been observed on land and in water but, unlike the Common Hippo, birth takes place on land. Young calves do not follow their mothers around but are left ‘parked’ by the mother in secluded pools with the mother returning at intervals to suckle (Galat-Luong 1981). This explains why tracks of young animals are hardly ever found (Hentschel 1990), because young start following their mothers at around 3–5 months of age when they are already quite large.

In general, the range of the Pygmy Hippo does not overlap with that of the Common Hippo but they may once have been sympatric in a few places in Liberia (Schomburgk 1913) and northwest Sierra Leone (Teleki and Baldwin 1980, Grubb et al. 1998). Roth et al. (2004) reported that in Côte d'Ivoire along the Bandama River, almost as far north as the confluence of the Nzi, where they were still found together with the Common Hippo in 1986.

The effects of natural predators on the pygmy hippo are unknown, but the principal carnivores capable of attacking an adult animal of this size are the Leopard Panthera pardus (Robinson 1970) and Nile Crocodile Crocodylus niloticus. Hentschel (1990) obtained a photograph of a juvenile Pygmy Hippo killed by a Leopard and Roth et al. (2004) saw two animals that had been mauled. Young animals are vulnerable to a wider range of predators, including African Golden Cat Caracal aurata, African Civet Civettictis civetta (Eltringham 1999) and African Rock Python Python sebae.

Although Pygmy Hippos are not generally a primary target for subsistence hunting, they are reported to be taken opportunistically by bushmeat hunters. There are also reports from some locations of Pygmy Hippos being targeted by commercial hunters (e.g. Dufour 2002). They are killed predominantly for their meat and, unlike the Common Hippo, their teeth have little trade value, but many of their body parts, including the skull, may be used in rituals or traditional medicine (Robinson 1970, Hentschel 1990).

The range of this species has changed markedly in the past 100 years (Robinson 1970, 1971, 2013), but most acutely in the past 50 years. Deforestation represents the biggest threat to Pygmy Hippos, with forests within their historical range having been steadily logged, farmed, converted to plantations (rubber, coffee and oil palm) and settled (Christie et al. 2007, Norris et al. 2010). An increase in mining and associated infrastructure development in recent years further threatens the remaining forests. What forest that does remain is fragmented, leaving Pygmy Hippo populations isolated, with demographic consequences and the increased susceptibility of small populations to local extinction (Mallon et al. 2011). Fragmentation has made the forests more accessible for hunters and there is now very little, if any, undisturbed forest in the region safe for wildlife causing the Pygmy Hippo to retreat into the diminishing fragmented parcels of forest (Lewison and Oliver 2008).

We quantified loss of forest in the range of the species using the Hansen et al. (2013) forest cover change maps. We used 300 m resolution maps (resampled from the original 30 m resolution to enable calculations to be computationally tractable). We calculated the approximate area of forest cover within the non-extinct portions of the species range in year 2000 (71,435 km2). We then summed the proportional forest loss (negative) and gain (positive) values between 2000-2012, which resulted in an estimate of a 5,010 km2 reduction in forest cover within the range. Note that this calculation assumes that each loss is a 100% loss and each gain is a 100% gain within a pixel. We then rescaled the net loss to a 26-year period (two generations; Pacifici et al. 2013). The result is a conservatively estimated ~15% net loss in forest cover over two generations.

Although Pygmy Hippos are not generally a primary target for subsistence hunting, they are reported to be taken opportunistically by bushmeat hunters. There are also reports from some locations of Pygmy Hippos being by commercial hunters (e.g. Dufour 2002). They are killed predominantly for their meat and, unlike the Common Hippo, their teeth have little trade value, but many of their body parts, including the skull, may be used in rituals or traditional medicine (Robinson 1970, Hentschel 1990).

Whilst Pygmy Hippos are legally protected in all range countries, the level of enforcement is limited due to a lack of capacity in terms of human and financial resources and adequate training.

The species is included on Appendix II of CITES (as Hexaprotodon liberiensis). It has full legal protection in all range state countries.

Remaining Pygmy Hippo populations are primarily found inside protected areas (Mallon et al. 2011). Effective protection and management of protected areas is therefore key to the persistence and conservation of pygmy hippos. The largest protected area containing Pygmy Hippos is Taï National Park and its Zone of Protection in Côte d’Ivoire. Other important sites are Sapo National Park, Grebo National Forest, Gola National Forest and Wonegizi proposed protected area in Liberia, the Gola Rainforest National Park, Tiwai Wildlife Sanctuary and Loma Mountains Forest Reserve in Sierra Leone, and Ziama Biosphere Reserve, Diécké and Mont Béro Reserves in Guinea. Due to the fragmented nature of these remaining populations it will be important to ensure corridors are maintained between sites.

In November 2010 the Pygmy Hippo sub-group of the IUCN SSC Hippo Specialist Group and the Zoological Society of London convened a workshop in Monrovia, Liberia, to develop a Regional Pygmy Hippo Conservation Strategy to guide conservation and research activities related to the Pygmy Hippo. This was attended by representatives from government agencies, local and international NGOs, academic institutions and Pygmy Hippo experts from all the range states and internationally. This strategy, published in 2011, identifies objectives and priority actions needed to address the threats to the pygmy hippos and halt their decline (Mallon et al. 2011).

In December 2012, the Liberia’s Forestry Development Authority and Fauna & Flora International hosted a national action planning meeting during which an Action Plan for the Conservation of the Pygmy Hippo in Liberia was produced. The action plan adopts the same Vision and Goal as the Regional Pygmy Hippo Conservation Strategy and articulates objectives necessary to attain these in Liberia and necessary actions required to achieve these (FFI and FDA 2013).

There are now conservation and/or research initiatives directly or indirectly targeting Pygmy Hippos in all four range countries. Details of these are found in the Regional Pygmy Hippo Conservation Strategy (Mallon et al. 2011).

Captive BreedingAs of 31 December 2013, the International Studbook for the Pygmy Hippopotamus records 353 living Pygmy Hippopotami kept in 133 institutions (Steck 2014). The age structure is, however, not very healthy. There is a lack of young animals and the sex ratio is skewed (38.53% males and 58.64% females, 2.83% unknown). 30-day mortality in both sexes is high: 33% in both sexes.